The advantages of optics for information transmission are generally well-known. Optics can provide more capacity and flexibility for large bandwidth communication than is achievable by using solely electronic components. The successful application of fiber optics demonstrates the advantages of optics for long-distance and wide-bandwidth telecommunication. As communication distance decreases and the number of communicating nodes simultaneously increases, there is a gradual shift from a telecommunication domain to an interconnection domain where typical applications are inter-and intra-computer communications. For the latter applications, low loss fibers become less critical components since free-space optics can provide comparable loss characteristics.
The use of free-space optics results in additional degrees of freedom since optical beams can be directed to traverse three-dimensional (3D) space system instead of being confined to a one-dimensional (1D) waveguide, such as a fiber.
Recent advances in developing micro-optic components has enabled progress in planar quasi-3D optical packaging for opto-electronic information processing systems. The flexibility to manipulate optical signals in the quasi-3D space provided by the planar optical geometry is not as good as that obtainable in a true 3D volume. However, the quasi-3D space system is more flexible than that achievable in a 1D waveguide or in a fiber. The quasi-3D systems offer more rigidity than that of a true free-space system. Schemes for performing optical signal distributions based on planar optics are described in an article by J. Jahns and A. Huang entitled "Planar Integration of Free-Space-Optical components" in Applied Optics, volume 28, pages 1602-1605, 1989. For performing more complicated operations, wavelength-division multiplexed (WDM) interconnects were proposed in an article by R. Linke entitled "Power Distribution in a Planar-waveguide-based Broadcast Star Network" in IEEE Photon. Tech. Lett., vol 3, pages 850 to 852, 1991 and in an article by S. Kawai and M. Mizoguchi entitled "Two-Dimensional Optical Buses for Massively Parallel Processing" in Optical Computing, vol 6, 1991, Technical Digest Series, at pages 136 to 139. Due to distribution loss and the complexity of the WDM device involved, the number of interconnecting nodes is very limited. It has also been suggested that a quasi-planar optical layout might be useful for relaying signals in a multistage network in an article by J. Jahns and B. Acklin entitled "Integrated Planar Optical Imaging System with High Interconnection Density" in Optics Letters, vol. 18, pages 1594 to 1596, October 1993 and by M. R. Feldman et al. entitled "Holographic Optical Interconnects for VLSI Multiclip Modules" in 42nd Electronic Components and Technology Conference 1992 (IEEE) vol. 1, pages 513 to 518. However, these schemes each require additional interconnects between stacks of such planar modules, making the design, fabrication, and alignment a very difficult task. A multistage network for a planar optical network implementation is described in an article by D. Nath et al. entitled "Efficient VLSI Networks for Parallel Processing Based on Orthogonal Trees", in IEEE Trans. Comput. C-32, pages 569-581, 1983.
In general, there are two important network parameters, the diameter of a network and the bisection-width of a network. The diameter of a network is the maximum distance between any pair of processors where the distance between a pair of processors is the minimum number of wires that must be traversed to travel from one processor to the other processor. The bisection-width of a network is the minimum number of wires that must be removed to separate the network into two halves with identical (within a difference of one processor) numbers of processors.
The simplest network of a set of N computers or processors is a ring. There are two major drawbacks associated with a ring. First, a ring has a large diameter so the number of hops between two processors is N in a worst case scenario. Second, a ring has a small bisection width since only two nodes need to be removed in order to divide the network into two equal halves. Therefore, a ring is slow in terms of message passing speed and has a small bandwidth.
Performance can be improved by adding more switching complexity to the network. In order to increase the speed, a tree structure can be used, thereby reducing the network diameter by a factor of (log.sub.2 N)/N or simply O[log(n)/N]. The symbol O(.largecircle.) refers to the complexity or upper bound of the asymptotic behavior of network. However, the bisection width of the tree topology remains small. Alternatively, in order to increase the capacity, i.e. to enlarge the bisection width, the connectivity must be increased. Similar to a ring, a nearest-neighbor rectangular mesh is a 2D version of a ring, but has a per-node connectivity two times larger than a single ring. A mesh is known to have a bisection width of .sqroot.N since .sqroot.N wires must be cut in order to decompose a mesh into two smaller but equal size meshes. The diameter of a mesh is also .sqroot.N, making a mesh a faster network than a ring but a slower network than a tree.
Almost all presently existing quasi-planar optical interconnection schemes rely upon multiple beam reflections inside the planar cavity. That is, in addition to the top reflecting plane, a portion of the bottom plane has to be used to reflect optical signals. Thus, the bottom surface has to be partitioned to interlace both the transmitting and reflecting components, thereby making fabrication and alignment even more difficult. In addition, as a result of using an extremely limited vertical dimension in planar optics, coupled with the limited field-of-view of manufacturable optical components, the required multiple reflections in many proposed interconnect schemes seriously limits the ratio of the active component area to the passive component area.